The Bug:

Arrow buttons within the book GUI are rendered above tooltips.

Steps to Reproduce:

1. Place down a command block and input the following command into it.

   /give @p minecraft:written_book{pages:['[{"text":"\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\nMC-75721\\n","color":"red","hoverEvent":{"action":"show_text","value":"MC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721"}}]','[{"text":"\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\nMC-75721\\n","color":"red","hoverEvent":{"action":"show_text","value":"MC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721\\nMC-75721 MC-75721"}}]'],title:MC-75721,author:MC-75721}

2. Activate the command block in order to receive a book.
3. Open the book and hold your mouse cursor over the red text displayed within it.
4. Take note as to whether or not arrow buttons within the book GUI are rendered above tooltips.

Observed Behavior:

Arrow buttons are rendered in front tooltips.

Expected Behavior:

Arrow buttons would be rendered behind tooltips.

Code Analysis:

Code analysis by Tobi14601 can be found in this comment.